In the manufacturing of a seamless pipe by the Mannesmann-mandrel mill process, first, a round billet or square billet is heated in a heating furnace and thereafter pierced and rolled by a piercing mill to manufacture a hollow blank pipe. Next, a mandrel bar is inserted into the inner face of the hollow blank pipe to be subjected to drawing and rolling by a mandrel mill including a plurality of roll stands. Thereafter, the pipe after drawing and rolling is rolled into a predetermined outer diameter by means of a reducing mill, thus providing a product.
As the mandrel mill described above, conventionally, widely used is a 2-roll type mandrel mill including a plurality of roll stands, in which two opposing grooved rolls are disposed in each roll stand, and the pressing directions of the grooved rolls are alternately shifted by 90° between adjacent roll stands.
In this 2-roll type mandrel mill, there is a risk that scoring may occur between a grooved roll and a blank pipe in the vicinity of a flange of the grooved roll caused by an excessive difference in circumferential speed between the groove bottom and the flange of the grooved roll, and a flaw (fin flaw) may occur in the blank pipe caused by excessive finning of the blank pipe material at a flange of the grooved roll. In view of preventing such scoring and fin flaws, in the 2-roll type mandrel mill, the grooved roll is generally designed such that the radius of curvature is larger at both ends of the groove profile (the groove shape obtained by sectioning the grooved roll with a plane that passes through the rotation center of the grooved roll). In this case, since the region of the blank pipe corresponding to the vicinity of the flange of the grooved roll is only subject to a tension in the longitudinal direction without being restricted either by the grooved roll or the mandrel bar, it is difficult to control the deformation (bulging) in the pipe circumferential direction. For this reason, a problem exists in that a pinhole defect etc. is likely to occur in a pipe made of a material having a low hot deformability such as a stainless steel.
To solve the above described problems of a 2-roll type mandrel mill, recently, a 3-roll type mandrel mill has become introduced in which three grooved rolls are disposed in each roll stand.
A typical 3-roll type mandrel mill includes a plurality of roll stands, in which three grooved rolls are disposed in each roll stand such that the angle formed by pressing directions is 120°, and the pressing directions of the grooved rolls are alternately shifted by 60° between adjacent roll stands.
In a typical 3-roll type mandrel mill, as described above, the pressing directions of the grooved rolls are alternately shifted by 60° between adjacent roll stands. Therefore, when wall thickness reduction is performed on the entire circumference of a blank pipe by a pair of adjacent roll stands, it is necessary to perform wall thickness reduction on a region of the blank pipe defined by a central angle of 60° per each grooved roll disposed in each roll stand (see FIG. 1B). In other words, the region where wall thickness reduction is not performed by each grooved roll is only the regions of the blank pipe defined by a central angle of 30° respectively corresponding to a region closer to opposite flanges of each grooved roll. Moreover, to perform wall thickness reduction on a region of the blank pipe defined by a central angle of 60°, the central angle defining a circular arc constituting a groove bottom profile (the profile in the vicinity of the grove bottom of a groove profile) of each grooved roll is set at 60° or more.
In contrast, in a 2-roll type mandrel mill, wall thickness reduction will be performed on a region of a blank pipe defined by a central angle of 90° per each grooved roll disposed in each roll stand (see FIG. 1A). In other words, the region where wall thickness reduction is not performed by each grooved roll is the region of the blank pipe defined by a central angle of 45° respectively corresponding to a region closer to opposite flanges of each grooved roll, and the range where wall thickness reduction is not performed is larger compared to the case of a typical 3-roll type mandrel mill.
Therefore, in the case of a typical 3-roll type mandrel mill, since the amount of outward bulge of the blank pipe material during drawing and rolling is smaller compared to the case of a 2-roll type mandrel mill, there is a risk that the circumference of the blank pipe is reduced due to drawing and rolling, and thereby the inner surface of the blank pipe squeezes the mandrel bar so that the mandrel bar becomes unable to be pulled out from a pipe after drawing and rolling.
To solve the problems of a typical 3-roll type mandrel mill as described above, Patent Literature 1 proposes a 3-roll type mandrel mill (claims of Patent Literature 1 etc.) in which the pressing directions of the grooved rolls are shifted by 40° for each roll stand among three roll stands which precede the final roll stand, and each grooved roll disposed in the above described three roll stands is formed so as to come into contact with a region of the blank pipe defined by a central angle of 40° (wall thickness reduction of the concerned region is performed).
To be specific, in the mandrel mill described in Patent Literature 1, the grooved roll disposed in the first and second roll stands is reported to be one which is used in a typical 3-roll type mandrel mill as shown in FIG. 3 of Patent Literature 1 etc. That is, the pressing directions of the grooved rolls are shifted by 60° between the first and second roll stands, and each grooved roll disposed in the first and second roll stands is configured to have a groove profile formed therein such that the grooved roll comes into contact with a region of the blank pipe defined by a central angle of 60° (wall thickness reduction is performed on the concerned region) (the central angle defining a circular arc constituting the groove bottom profile is set at 60°).
Further, the mandrel mill described in Patent Literature 1 is configured such that the pressing directions of the grooved rolls are shifted by 40° for each roll stand among a third to a fifth roll stands, and each grooved roll disposed in the third to fifth roll stands has a groove profile formed therein such that the grooved roll comes into contact with a region of the blank pipe defined by a central angle of 40° (wall thickness reduction is performed on the concerned region) (the central angle defining the circular arc constituting the groove bottom profile is set at) 40°.
In other words, in the mandrel mill described in Patent Literature 1, the region where wall thickness reduction is not performed by each grooved roll disposed in the third to fifth roll stands is a region of the blank pipe defined by a central angle of 40° respectively corresponding to the region closer to opposite flanges of each grooved roll, and the range where wall thickness reduction is not performed is larger compared with the case of a typical 3-roll type mandrel mill. Therefore, in the mandrel mill described in Patent Literature 1, the amount of outward bulge of the blank pipe material during drawing and rolling will become larger in the third to fifth roll stands compared to the case of a typical 3-roll type mandrel mill.